/Engine/Source/Editor/UnrealEd/Public/EditorWorldExtension.h
Checks if the passed extension already exists and creates one if it doesn't.
[UEditorWorldExtension](API\Editor\UnrealEd\UEditorWorldExtension) * AddExtension ( [TSubclassOf](API\Runtime\CoreUObject\Templates\TSubclassOf)< [UEditorWorldExtension](API\Editor\UnrealEd\UEditorWorldExtension) > EditorExtensionClass )